Optical waveguides may be formed within substrates and transmit light waves. The light waves are kept in the optical waveguide by total internal reflection at interfaces between the optical waveguide and the substrate. The total internal reflection is caused mainly by a refractive index difference between the optical waveguide and the substrate. The refractive index difference between the optical waveguide and the substrate is often less than satisfactory and, thus, a part of the light wave may leak out of the optical waveguide, increasing insertion loss of the optical waveguide. Ridge-type optical waveguides (i.e., ridge waveguides) have three sides interfaced with the air and a refractive index difference between the optical waveguide and the air is often larger than satisfactory. Therefore, leakage of the light waves from the ridge waveguide can be reduced.